1. Field of the Invention
The present invention relates to an overcharge inhibitor, and a nonaqueous electrolyte solution and a secondary battery using the same.
2. Description of Related Art
A lithium-ion secondary battery containing a nonaqueous electrolyte solution (a nonaqueous electrolyte solution secondary battery) has been widely used in the field of portable information equipment and the like because of high voltage (operating voltage of 4.2 V) and high energy density. The demands for such a lithium-ion battery have grown rapidly. Currently, the lithium-ion secondary battery establishes the position of a standard cell for a mobile information equipment including a cellular phone and a notebook computer.
The lithium-ion secondary battery includes a positive electrode, a negative electrode and a nonaqueous electrolyte solution as components. In particular, the general lithium secondary battery uses a lithium composite metal oxide typified by LiMO2 (where M contains one or more kinds of metal elements selected from the group consisting of Co, Ni and Mn) as the positive electrode. Further, the lithium secondary battery uses a carbon material or an intermetallic compound containing Si, Sn or the like as the negative electrode, and a nonaqueous solution dissolving an electrolyte salt in a nonaqueous solvent (organic solvent) as the electrolyte solution.
The nonaqueous solvents generally used include carbonates, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and diethyl carbonate (DEC).
When such a lithium secondary battery is overcharged to exceed the normal operating voltage (for example, 4.2 V in full charge in the case of LiCoO2), excessive lithium ions are extracted from the positive electrode, and excessive lithium is deposited in the negative electrode to form dendrite at the same time. Both electrodes, namely, the positive and negative electrodes chemically become unstable to gradually react with the carbonates in the nonaqueous solution, and cause a sudden exothermal reaction by decomposition or the like. Thus, the entire battery abnormally generates heat, which disadvantageously impairs the safety of the battery.
Normally, measures are taken to prevent the overcharge by use of a protective circuit or the like so as not to cause an internal short-circuit, which does not lead to the abnormal state. However, a battery charger or the protective circuit may be broken down. In case of breakdown, the safety of the battery is required even when the battery itself is overcharged. In particular, this problem becomes important as the energy density and capacity of the battery increases.
In order to solve such a problem, a technique is proposed which ensures the safety of the battery against the overcharge by adding a small amount of an aromatic compound as an additive to an electrolyte solution as disclosed in Japanese Patent Publication No. 3275998 (Patent Document 1), Japanese Patent Laid-openNo. Hei 09 (1997)-171840 (Patent Document 2), Japanese Patent Laid-open No. Hei 10 (1998)-321258 (Patent Document 3), Japanese Patent Laid-open No. Hei 07 (1995)-302614 (Patent Document 4), and “Electrochemical and Solid-State Letters, 9 (1), A24-A26 (2006)” (Non-Patent Document 1).
The techniques disclosed in Patent Documents 1 to 3 are designed to use cyclohexyl benzene, biphenyl, 3-R-thiophene, 3-chlorotiophene, furan or the like dissolved in an electrolyte solution so as to ensure the safety of the lithium-ion battery itself in overcharging. The use of such an electrolyte solution generates gas in the battery at the time of overcharge to operate an internal electricity breaking device, or generates a conductive polymer or the like, thus preventing the overcharge of the battery.
Patent Document 4 discloses a nonaqueous electrolyte secondary battery including an organic compound with a π-electron orbit which has a molecular weight of 500 or less, and which has a reversible oxidation reduction potential nobler than the positive electrode potential in full charge of the battery. In Patent Document 4, an anisole derivative or the like is exemplified as the above organic compound.
Non-Patent Document 1 discloses the technique that the overcharge can be suppressed by adding a thiophene based polymer having an electric activity and having about several thousands of molecular weight to an electrolyte solution. The polymers include, for example, poly(3-butylthiophene), and poly(3-phenylthiophene).